1. Field of the Invention
This invention relates to a method for restricting fluid flow through the medium to high permeable strata of subterranean reservoirs having heterogeneous permeability at a substantial depth outwardly from a bore hole. More particularly, the invention relates to such a method to provide better control of fluids subsequently injected into a reservoir during enhanced oil recovery operations or withdrawn from a reservoir during production operations.
2. Description of the Prior Art
When fluids flow through reservoirs having sections of varying permeability, a higher percentage of the fluids tends to flow through those sections having a higher permeability. It is often desired to decrease or stop the flow of fluids through these sections of higher permeability.
For example, in the enhanced recovery of petroleum by flooding, a displacing fluid is injected into the reservoir via one or more injection wells to displace the petroleum through the reservoir toward one or more producing wells.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid builds up in a wide band in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact, and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the reservoir. If this uniform permeability does not exist, or is not provided, the flooding fluid will follow the path of least resistance, pass mostly through the portions of the reservoir having the highest permeability and bypass the petroleum present in the less permeable portions of the reservoir. This results in the loss of some driving fluid energy and the early appearance of excessive amounts of driving fluid at the producing well. If fluid flow through these high permeability zones of the reservoir was restricted or they were plugged, the injected fluid would be forced to flow into the less permeable portions of the reservoir and displace a higher percentage of the petroleum present in the entire reservoir. Similarly, in the production of oil, producing wells sometimes produce water and/or gas along with oil. The water and gas often are produced through the portions of the reservoir having a relatively high permeability. If the zones through which water and gas are produced could be at least partially plugged, a higher percentage of the produced fluids would be the desired oil phase.
A wide variety of materials have been proposed for use in plugging subterranean reservoirs. It is known to inject an aqueous solution of a water-soluble polymer and a cross-linking agent which reacts in the reservoir to form a plug. Similarly, plugs have been formed using aqueous solutions of sodium silicate and a gelling agent therefor. U.S. Pat. No. 4,069,869 to Sandiford described a method for forming a mixed plug in a reservoir by injecting aqueous solutions of: (1) a polymer such as polyacrylamide, polysaccharide or a cellulose ether, (2) a cross-linking agent such as sodium dichromate which reacts with the polymer to form a time-delayed polymer-containing plug, (3) an alkali metal silicate such as sodium silicate, and (4) a gelling agent such as ammonium sulfate which reacts with the silicate to form a silicate-containing plug. The injected compositions are either admixed at the surface or injected simultaneously. U.S. Pat. No. 4,009,755 to Sandiford discloses a method for forming a combination plug using materials similar to those described in the previous Sandiford patent wherein there is injected into a reservoir aqueous solutions of a polymer and a cross-linking agent which react to form a first plug followed by an aqueous solutions of an alkali metal silicate and a gelling agent which react to form a second plug.
The various plug-forming compositions and methods previously suggested have met with some success, especially in reducing fluid flow through the highest permeability channels of heterogeneous reservoirs which require formation of a very stiff gel or solid, i.e., the type of plug best provided by a plug-forming composition including either a cross-linking agent or a gelling agent. However, need remains for even more effective methods for forming plugs, especially in reservoirs having channels or zones of high to medium permeability where a very stiff gel or solid plug is not required or sometimes not even desired. A stiff gel or solid plug may adequately plug a portion of a very high permeability channel, but, after formation, the plug has little ability to continue to move or flow and may be bypassed by other low viscosity fluids flowing through the reservoir. Thus, it has been difficult to form in the higher permeability channels of a reservoir a material which restricts fluid flow through that portion of the reservoir, but which fluid flow-restricting material remains capable of at least limited flow itself when occupying a channel to which a pressure differential is applied so that it is not so easily bypassed by other fluids.
One especially troublesome type of reservoir of heterogeneous permeability in which to carry out enhanced recovery operations is one in which any injected treating fluid, such as a plug-forming composition, takes a relatively long period of time to travel from an injection well to a production well. Travel time can be as long as one month, two months or even longer. High permeability channels often exist over the entire distance between wells. Plug-forming compositions containing cross-linking agent and/or a gelling agent tend to form plugs within a relatively short period of time, as within 24 hours after being injected. Also many cross-linking agents and gelling agents exhibit a tendency to adsorb onto reservoir rock so that such compositions which have passed through the reservoir only a limited distance away from the injection bore hole become partially, substantially or completely depleted of cross-linking agent or gelling agent. Thus, due to several factors it is difficult to form a plug a substantial distance from the injection bore hole.
Accordingly, a principal object of this invention is to provide a method for controlling the permeability of a subterranean formation of nonuniform permeability.
Another object of the invention is to provide a method for reducing channeling of a flooding medium from an injection well to a producing well via a zone of high to medium permeability.
Still another object of the invention is to provide a method for selectively restricting fluid flow through water-producing and gas-producing zones in a subterranean reservoir.
A further object of the invention is to restrict fluid flow through the higher to medium permeability zones of a reservoir at a substantial distance from the injection bore hole.
A still further object of the invention is to achieve an in depth restriction of fluid flow through the medium to high permeability zones of a reservoir.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.